Mineral fouling is a common occurrence in a variety of residential, commercial, and industrial operations when substrates are exposed to naturally occurring or so-called “tap” water. Some of the more recognizable examples include residential water heaters, residential shower heads, commercial HVAC systems having cooling towers or heat exchangers, heat exchangers in the chemical process industry, and oil and gas extraction operations. Mineral fouling, or the formation of mineral scale, is a result of the precipitation of salts dissolved in the water onto the substrate. Mineral scale can take the form of an accumulation of relatively soft deposits, generally caused by the deposition of minerals that precipitate in the bulk fluid. Soft scale is typically easy to remove. However, the solubility of dissolved salts decreases with temperature. Therefore, when the surface of a heated substrate is exposed to water containing dissolved salts, salts can directly precipitate onto the surface. Such directly precipitated deposits, which often comprise a combination of many different minerals including calcium sulfate and calcium carbonate, are extremely durable and resist removal via mechanical abrasion and/or dissolution by acids. As deposits accumulate on equipment surfaces, such as heat exchangers, the resistance to heat transfer increases, which results in decreased energy efficiency. It has been estimated that up to 0.25% of gross domestic product value is lost because of the inefficiencies in process equipment introduced by mineral scaling.
Numerous technologies have been developed to resist mineral scaling. Most of these conventional technologies rely on additives, introduced into the processing of water, to inhibit precipitation. However, these additives must be constantly supplied and replenished and may constitute contaminants in wastewater streams requiring expensive post-treatment processes to mitigate environmental concerns. More sophisticated techniques to reduce scaling include electromagnetic devices that expose a portion of the process water to a large magnetic field. The magnetic field is designed to disturb the motion of ions in the process water and, thereby, promote precipitation. By precipitating salts away from critical process equipment, the amount of subsequent precipitation to form mineral scale on critical equipment surfaces can be reduced. Such electromagnetic devices require a continuous power supply, are expensive to fabricate and maintain, and require additional space for installation, operation, and maintenance.
Yet another conventional, more elegant solution to mineral scaling is development of surface treatments or coatings that discourage or eliminate the growth of scale. As used herein, a “treatment” means application of a chemical compound to the surface of a substrate so as to form a deposit thereon that is not a mechanically-distinct layer overlying the surface and cannot exhibit layer behavior, such as delamination or peeling. As used herein, a “coating” means deposition of a chemical compound to the surface of a substrate so as to form a mechanically discrete layer that exhibits layer behavior, such as, a capacity of being peeled off. In the absence of information about mechanical properties, a layer having a thickness of less than about 10 nm will be considered a “treatment” while a layer having a thickness greater than about 10 nm will be considered a “coating.” Conventional treatments and/or coatings that inhibit the growth of mineral scale include hydrophobic coatings, such as DuPont's Teflon® coatings, or hydrophilic coatings. In systems using mixtures of oil and water as a process fluid, the coatings that wet oil and that do not absorb water are preferred for resisting the growth of mineral scale. One such conventional coating includes an oil that is immiscible with water and prevents contact with the aqueous phase comprising the dissolved minerals. Still other conventional solutions include manufacture of surfaces having an energy of formation per unit area of less than about 32 mJ/m2 and/or a ratio of polar-to-total energy of formation of less than 0.2. Such surfaces resist the accumulation of mineral scale in static environments due to a presumed decrease in the nucleation of mineral salt crystals on the surface of a substrate (e.g., microscope slides).
These conventional treatments and/or coatings are limited to resisting an initiation of mineral scale growth. If a material scale deposit begins, such as at a defect in the coating or treatment, then resistance to further accumulation of material scale is lost. Therefore, these conventional treatments or coatings lack the continuous counteraction provided by chemical or electromagnetic water treatment methods, described previously. Because the coatings on materials used in residential, commercial, and industrial environments (as opposed to laboratory environments) are likely to include defects, the practical value of coatings and/or treatments may be limited.
Despite these various advancements in the field of mineral scaling, there remains a need for improved methods by which the accumulation of mineral scale can be resisted. It would be desirable for such improved methods to combine the elegance of a simple surface treatment or coating with the continuous counteraction provided by chemical or electromagnetic water treatments and that may be realized by application of a thin-layer treatment or coating without a substantial increase in thermal resistance (generally speaking, the thermal resistance is proportional to a layer thickness divided by a thermal conductivity).
Fundamental studies of the adhesion of solid objects to polymeric coatings, such as the ice adhesion study of Meuler et al., ACS Appl. Mater. Interfaces, Vol. 2, pp. 3100-3110 (“Meuler”) have shown that, when very thin coatings on rigid substrates are used, the “practical work of adhesion” decreases as the receding contact angle of the surface in contact with an appropriate probe liquid increases. The force needed to remove a solid object from such a surface is proportional to the practical work of adhesion. As a result, the receding contact angle measured using an appropriate probe liquid provides a reliable indication of the relative amount of force (per unit area) needed to remove a solid object from the surface. For the adhesion of ice, liquid water is clearly the most appropriate probe liquid. For other substances, however, such as adhered metal, the use of a molten form of the solid as a probe liquid is not practical, as it may require temperatures that would destroy the coating.
Moreover, it is well known to those skilled in the art of adhering surfaces that the roughness of a surface is a critical determinant of adhesion. Smooth surfaces are known to have significantly lower adhesion than rough surfaces. Generally speaking, a rougher surface provides a greater true surface area than a smooth surface, thus increasing the total force required to remove an object from said surface even when the “work of adhesion,” which is the energy required per unit of true surface area, remains the same. Furthermore, roughening of surfaces provides locations where mechanical interlocking of the surface and an adhered object may occur. Removal of the solid object then requires overcoming the mechanical interlocking forces in addition to overcoming the work of adhesion. Conveniently, the receding contact angle of a fully wetted surface is known to decrease with increasing roughness. Thus, as an indicator of adhesion, the receding contact angle measurement accounts not only for chemical interactions between surfaces, but also for roughness.
In addition to roughness, the physical and chemical heterogeneity of a surface also contribute to the adhesion of solid objects. In particular, heterogeneous surfaces contain locations of increased affinity between the surface and an adhered solid object, due either to chemical species or to physical topography that “pins” an adhered object to the surface. These heterogeneities may be considered as “pinning defects.” The greater the number and tenacity of these pinning defects, the greater the adhesion of a solid object to a surface will be. Again, conveniently, the presence of pinning defects tends to increase the difference between advancing and receding contact angles on a surface. Because the receding contact angle is, in all but exceptionally rare cases, smaller than the advancing contact angle, the presence of pinning defects will decrease the receding contact angle as it increases the difference between the advancing and receding angle. Therefore, the receding contact angle also captures effects due to pinning defects on surface adhesion.
Beyond the aforementioned effects, it is also possible for the molecular fragments present on many surfaces to reorganize into new physical arrangements upon contact with liquids and solids. Such rearrangements almost always draw molecular fragments with an affinity for the adhering substance closer to the surfaces, but push away fragments with a lower affinity. Such rearrangements therefore increase the adhesion of solid and liquid objects to surfaces. Again, conveniently, this phenomenon leads to a difference between advancing and receding contact angles as measured on a surface. As described before, the net result is that surfaces with a greater capacity to reorganize in contact with a particular liquid will demonstrate a lower receding contact angle. As an indicator of adhesion, therefore, receding contact angles also capture effects due to surface reorganization.
Surfaces having a terminal —CF3 molecular fragment at the outermost molecular layer are known to have generally the highest contact angles with water, the lowest surface energies, and the lowest levels of adhesion and frictional interaction with solids and liquids. When these —CF3 fragments are immediately adjacent to fragments other than —CF2— linking fragments, dipolar forces result that increase adhesion. Moreover, surfaces with —CF3 fragments connected to a small number of, or no, adjacent —CF2— linking fragments are prone to reorganization. It is therefore known that, to produce a surface having minimal adhesion to solid or liquid objects, a well-ordered array of —CF3 fragments adjacent to as many —CF2-linking fragments as possible, having minimal roughness and minimal defects, is required. Although methods of chemical modification, for instance with silanes, are known to produce such surfaces, at present, general methods and compositions for producing such surfaces using discrete molecules, without chemical modification of an underlying substrate, are not known. Such methods and compositions must satisfy the following requirements: 1) allow for self-assembly of a well-ordered monolayer of —CF3 fragments adjacent to numerous —CF2-linking fragments within a practical time frame, 2) maintain this arrangement under conditions of operation for a long enough period to be of practical use, 3) prohibit naturally occurring surface rearrangements (such as the growth of large crystalline domains) that produce pinning defects, and 4) produce surfaces with the minimum possible roughness.